1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor structure for two-dimensional magnetic recording (TDMR).
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu) or silver (Ag). One ferromagnetic layer adjacent to the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference or pinned layer. The other ferromagnetic layer adjacent to the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the pinned-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the amount of tunneling current through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
A proposed technology that uses multiple CPP-MR sensors is two-dimensional magnetic recording (TDMR). In TDMR, multiple sensors that are located on a single structure access the same or adjacent data tracks to obtain signals that are processed jointly. This allows the data tracks to be placed closer together, resulting in an increase in areal data bit density. In addition to increasing areal density, TDMR may provide an increased readback data rate if data from multiple data tracks are read concurrently. A structure with multiple stacked read sensors for TDMR is described in US 2013/0286502 A1.
Each of the individual CPP-MR sensors in a TDMR read head structure is required to be located between two shields of magnetically permeable material that shield the sensors from recorded data bits that are neighboring the data bit being read. During readback, the shields ensure that each sensor reads only the information from the targeted disk region.
In a TDMR sensor structure, such as a structure with two stacked read sensors, a problem arises if the free layer of the lower read sensor has its magnetization magneto-statically biased by side shields of soft magnetic material. An antiferromagnetic layer is needed to pin the magnetization of the center shield in a direction substantially parallel to the ABS. Since the center shield is ferromagnetically exchange coupled to the side-shields of the lower sensor their magnetization is pinned substantially parallel to the ABS as well, assuring the stabilization of the free layer of the lower sensor. Because the reference or pinned layers of the two read sensors also have their magnetizations pinned by antiferromagnetic layers, but in a direction substantially perpendicular to the ABS, i.e., orthogonal to the magnetization of the center shield, at least two separate annealing steps are required. A first annealing step at high temperature is performed after formation of the upper sensor to pin the magnetizations of the lower and upper sensors' pinned layers substantially orthogonal to the ABS. This necessarily also pins the magnetization of the center shield orthogonal to the ABS. Thus after formation of the top shield above the upper sensor, a second annealing step at lower temperature is required to reset the magnetization of the center shield to be parallel to the ABS and to pin the magnetization of the top shield to be parallel to the ABS. The lower temperature is required so as to not disturb the pinned magnetizations of the upper and lower sensor which have previously been set at higher temperature. However, due to its lower temperature this second annealing step does not fully reset the magnetization of the center shield back to parallel to the ABS, which adversely affects the stabilization of the lower sensor.
What is needed is a stacked CPP-MR sensor structure for TDMR that has a center shield with magnetization fully aligned parallel to the ABS.